Time varying cyclic delay diversity of OFDM

ABSTRACT

Methods and apparatuses that apply a time-varying delay to symbols to be transmitted from one or more antennas are provided.

CROSS REFERENCE

This application claims priority from Provisional Application No.60/572,137, filed May 17, 2004, entitled “Systems Time Varying CyclicDelay Diversity of OFDM” and is assigned to the assignee of the presentapplication.

BACKGROUND

1. Field

The present document relates generally to wireless communication, andamongst other things to, signal transmission in multi-antenna system.

2. Background

In a wireless communication system, an RF modulated signal from atransmitter may reach a receiver via a number of propagation paths. Thecharacteristics of the propagation paths typically vary over time due toa number of factors such as fading and multipath. To provide diversityagainst deleterious path effects and improve performance, multipletransmit and receive antennas may be used. A multiple-inputmultiple-output (MIMO) communication system employs multiple (N_(T))transmit antennas and multiple (N_(R)) receive antennas for datatransmission. A MIMO channel formed by the N_(T) transmit and N_(R)receive antennas may be decomposed into N_(S) independent channels, withN_(S)≦min {N_(T), N_(R)}. Each of the N_(S) independent channels mayalso be referred to as a spatial subchannel (or a transmission channel)of the MIMO channel and corresponds to a dimension.

If the propagation paths between the transmit and receive antennas arelinearly independent (i.e., a transmission on one path is not formed asa linear combination of the transmissions on the other paths), which isgenerally true to at least an extent, then the likelihood of correctlyreceiving a data transmission increases as the number of antennasincreases. Generally, diversity increases and performance improves asthe number of transmit and receive antennas increases

To further improve the diversity of the channels a transmit diversitytechnique may be utilized. Many transmit diversity techniques have beenexplored. One such technique is transmit delay diversity. In transmitdelay diversity a transmitter utilizes two antennas that transmit thesame signal, with the second antenna transmitting a delayed replica ofthat transmitted by the first antenna. By so doing, the second antennacreates diversity by establishing a second set of independent multipathelements that may be collected at the receiver. If the multipathgenerated by the first transmitter fades, the multipath generated by thesecond transmitter may not, in which case an acceptable Signal-To-NoiseRatio (SNR) will be maintained at the receiver. This technique is easyto implement, because only the composite TX0+TX1 channel is estimated atthe receiver. The biggest drawback to transmit delay diversity is thatit increases the effective delay spread of the channel, and can performpoorly when the multipath introduced by the second antenna falls upon,and interacts destructively with, the multipath of the first antenna,thereby reducing the overall level of diversity.

To deal with standard delay diversity problems, additional delaydiversity techniques have been developed. One such technique is referredto as cyclic delay diversity. A cyclic delay is one where the samples ofeach symbol of the n_(i) symbols are shifted in the order in which theyare transmitted as part of the symbol. Those samples that are beyond theeffective part of the symbol are transmitted in the beginning of thatsymbol. In such a technique, a prefix is pre-pended to each sample thatfixes a delay, or order, for transmitting the sample from the specificantenna as part of the symbol. The cyclic delays allow for longerdelays, however, which would otherwise be limited to fractions of theguard interval period to avoid inter-symbol interference.

A cyclic delay diversity scheme may introduce frequency selectivity inthe channel and hence may provide diversity benefit for flat channels.It does not provide, however, any time diversity when the channel is notin and of itself time selective. For example, if two transmit antennasare in slow fading or static channels, the cyclic shift Δ_(m) may besuch that the two channels, e.g. H₁(n) and H₂(n), add up destructively(or constructively) all the time.

Therefore, it is desired to provide a delay diversity scheme whichminimizes the possibility of destructive or constructive addition of thechannels utilized to provide diversity.

SUMMARY

In one aspect, a method for providing transmission diversity comprisesproviding, to a first antenna, a first symbol after a first delayperiod, providing, to the first antenna, a second symbol after a seconddelay period that is different than the first delay period, andproviding, to the first antenna, a third symbol after a third delayperiod that is different than the first delay period and the seconddelay period.

In another aspect, a transmitter comprises at least two antennas, amodulator, and a delay circuit that delays symbols output from themodulator to the antenna by a delay period that varies over time.

In an additional aspect, a wireless transmitter comprises at least twoantennas and a that stores a plurality of symbols each comprising aplurality of samples, wherein the memory outputs the plurality ofsamples of a first symbol after a first delay to one antenna of the atleast two antennas and a second symbol of the plurality of symbols aftera second delay to the one antenna. The first delay and the second delayare different.

In a further aspect, a transmitter comprises at least three antennas, amodulator, a first delay circuit coupled between the modulator and oneof the at least two antennas, the first delay circuit delaying symbolsoutput from the modulator to the antenna by a delay period that variesover time, and a second delay circuit coupled between the modulator andanother of the at least two antennas, the first delay circuit delayingsymbols output from the modulator to the another antenna by a anotherdelay period that varies over time. The another delay period and thedelay period are different.

In yet another aspect, a method for providing transmission diversity ina multi-channel communication system comprises applying a first phaseshift to a first symbol to be transmitted on a first antenna andapplying a second phase shift, different than the first phase shift, tothe first symbol to be transmitted on a second antenna.

In yet a further aspect, a transmitter comprises at least two antennas,a modulator, and a phase shift that applies a phase shift to symbolsoutput by the modulator to the antenna by a phase shift that varies overtime.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates a block diagram of an embodiment of a transmittersystem and a receiver system in a MIMO system;

FIG. 2 illustrates a block diagram of an embodiment of a transmitterunit that provides time-varying delay diversity;

FIG. 3 illustrates an embodiment of a time varying delay applied tosymbols transmitted from a same antenna;

FIG. 4 illustrates an embodiment of a time varying delay applied to asymbol transmitted on multiple antennas;

FIG. 5 illustrates a block diagram of another embodiment of atransmitter unit that provides time-varying delay diversity;

FIG. 6 illustrates a block diagram of an embodiment of a receiver unitcapable of utilizing time-varying delay diversity;

FIG. 7 illustrates a block diagram of an embodiment of a delay element;

FIG. 8 illustrates a flow chart of an embodiment of a method forproviding time-varying diversity;

FIG. 9 illustrates a block diagram of a further embodiment of atransmitter unit that provides time-varying delay diversity; and

FIG. 10 illustrates a flow chart of a further embodiment of a method forproviding time-varying diversity.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments and isnot intended to represent the only embodiments in which the presentinvention can be practiced. The term “exemplary” used throughout thisdescription means “serving as an example, instance, or illustration,”and should not necessarily be construed as preferred or advantageousover other embodiments. The detailed description includes specificdetails for the purpose of providing a thorough understanding of thepresent invention. However, it will be apparent to those skilled in theart that the present invention may be practiced without these specificdetails. In some instances, well known structures and devices are shownin block diagram form in order to avoid obscuring the concepts of thepresent invention.

Multi-channel communication systems include multiple-inputmultiple-output (MIMO) communication systems, orthogonal frequencydivision multiplexing (OFDM) communication systems, MIMO systems thatemploy OFDM (i.e., MIMO-OFDM systems), and other types of transmissions.For clarity, various aspects and embodiments are described specificallyfor a MIMO system.

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission. A MIMO channel formed bythe N_(T) transmit and N_(R) receive antennas may be decomposed intoN_(S) independent channels, with N_(S)≦min {N_(T), N_(R)}. Each of theN_(S) independent channels may also be referred to as a spatialsubchannel (or transmission channel) of the MIMO channel. The number ofspatial subchannels is determined by the number of eigenmodes for theMIMO channel, which in turn is dependent on a channel response matrix,H, that describes the response between the N_(T) transmit and N_(R)receive antennas. The elements of the channel response matrix, H, arecomposed of independent Gaussian random variables {h_(i,j)}, for i=1, 2,. . . N_(R) and j=1, 2, . . . N_(T), where h_(i,j) is the coupling(i.e., the complex gain) between the j-th transmit antenna and the i-threceive antenna. For simplicity, the channel response matrix, H, isassumed to be full-rank (i.e., N_(S)=N_(T)≦N_(R)), and one independentdata stream may be transmitted from each of the N_(T) transmit antennas.

FIG. 1 is a block diagram of an embodiment of a transmitter system 110and a receiver system 150 in a MIMO system 100. At transmitter system110, traffic data for a number of data streams is provided from a datasource 112 to a transmit (TX) data processor 114. In an embodiment, eachdata stream is transmitted over a respective transmit antenna. TX dataprocessor 114 formats, codes, and interleaves the traffic data for eachdata stream based on a particular coding scheme selected for that datastream to provide coded data.

The coded data for each data stream may be multiplexed with pilot datausing, for example, time division multiplexing (TDM) or code divisionmultiplexing (CDM). The pilot data is typically a known data patternthat is processed in a known manner (if at all), and may be used at thereceiver system to estimate the channel response. The multiplexed pilotand coded data for each data stream is then modulated (i.e., symbolmapped) based on a particular modulation scheme (e.g., BPSK, QSPK,M-PSK, or M-QAM) selected for that data stream to provide modulationsymbols. The data rate, coding, and modulation for each data stream maybe determined by controls provided by a processor 130.

The modulation symbols for all data streams are then provided to a TXMIMO processor 120, which may further process the modulation symbols(e.g., for OFDM). TX MIMO processor 120 then provides N_(T) modulationsymbol streams to N_(T) transmitters (TMTR) 122 a through 122 t. Eachtransmitter 122 receives and processes a respective symbol stream toprovide one or more analog signals, and further conditions (e.g.,amplifies, filters, and upconverts) the analog signals to provide amodulated signal suitable for transmission over the MIMO channel. N_(T)modulated signals from transmitters 122 a through 122 t are thentransmitted from N_(T) antennas 124 a through 124 t, respectively.

At receiver system 150, the transmitted modulated signals are receivedby N_(R) antennas 152 a through 152 r, and the received signal from eachantenna 152 is provided to a respective receiver (RCVR) 154. Eachreceiver 154 conditions (e.g., filters, amplifies, and downconverts) arespective received signal, digitizes the conditioned signal to providesamples, and further processes the samples to provide a corresponding“received” symbol stream.

An RX MIMO/data processor 160 then receives and processes the N_(R)received symbol streams from N_(R) receivers 154 based on a particularreceiver processing technique to provide N_(T) “detected” symbolstreams. The processing by RX MIMO/data processor 160 is described infurther detail below. Each detected symbol stream includes symbols thatare estimates of the modulation symbols transmitted for thecorresponding data stream. RX MIMO/data processor 160 then demodulates,deinterleaves, and decodes each detected symbol stream to recover thetraffic data for the data stream. The processing by RX MIMO/dataprocessor 160 is complementary to that performed by TX MIMO processor120 and TX data processor 114 at transmitter system 110.

RX MIMO processor 160 may derive an estimate of the channel responsebetween the N_(T) transmit and N_(R) receive antennas, e.g., based onthe pilot multiplexed with the traffic data. The channel responseestimate may be used to perform space or space/time processing at thereceiver. RX MIMO processor 160 may further estimate thesignal-to-noise-and-interference ratios (SNRs) of the detected symbolstreams, and possibly other channel characteristics, and provides thesequantities to a processor 170. RX MIMO/data processor 160 or processor170 may further derive an estimate of the “operating” SNR for thesystem, which is indicative of the conditions of the communication link.Processor 170 then provides channel state information (CSI), which maycomprise various types of information regarding the communication linkand/or the received data stream. For example, the CSI may comprise onlythe operating SNR. The CSI is then processed by a TX data processor 178,modulated by a modulator 180, conditioned by transmitters 154 a through154 r, and transmitted back to transmitter system 110.

At transmitter system 110, the modulated signals from receiver system150 are received by antennas 124, conditioned by receivers 122,demodulated by a demodulator 140, and processed by a RX data processor142 to recover the CSI reported by the receiver system. The reported CSIis then provided to processor 130 and used to (1) determine the datarates and coding and modulation schemes to be used for the data streamsand (2) generate various controls for TX data processor 114 and TX MIMOprocessor 120.

Processors 130 and 170 direct the operation at the transmitter andreceiver systems that they are coupled with including the appropriatetransmit and receive data processors. Memories 132 and 172 providestorage for program codes and data used by processors 130 and 170,respectively.

The model for an OFDM MIMO system may be expressed as:y=Hx+n,  Eq (1)

-   -   where y is the received vector, i.e., y=[y₁ y₂ . . . y_(N) _(R)        ]^(T), where {y_(i)} is the entry received on the i-th received        antenna and i ε{1, . . . , N_(R)};    -   x is the transmitted vector, i.e., x=[x₁ x₂ . . . x_(N) _(T)        ]^(T), where {x_(j)} is the entry transmitted from the j-th        transmit antenna and j ε{1, . . . , N_(T)};    -   H is the channel response matrix for the MIMO channel;    -   n is the additive white Gaussian noise (AWGN) with a mean vector        of 0 and a covariance matrix of Λ_(n)=σ²I, where 0 is a vector        of zeros, I is the identity matrix with ones along the diagonal        and zeros everywhere else, and σ² is the variance of the noise;        and    -   [.]^(T) denotes the transpose of [.].        Due to scattering in the propagation environment, the N_(T)        symbol streams transmitted from the N_(T) transmit antennas        interfere with each other at the receiver. In particular, a        given symbol stream transmitted from one transmit antenna may be        received by all N_(R) receive antennas at different amplitudes        and phases. Each received signal may then include a component of        each of the N_(T) transmitted symbol streams. The N_(R) received        signals would collectively include all N_(T) transmitted symbols        streams. However, these N_(T) symbol streams are dispersed among        the N_(R) received signals.

At the receiver, various processing techniques may be used to processthe N_(R) received signals to detect the N_(T) transmitted symbolstreams. These receiver processing techniques may be grouped into twoprimary categories:

-   -   spatial and space-time receiver processing techniques (which are        also referred to as equalization techniques), and    -   “successive nulling/equalization and interference cancellation”        receiver processing technique (which is also referred to as        “successive interference cancellation” or “successive        cancellation” receiver processing technique).

FIG. 2 is a block diagram of a portion of a transmitter unit 200, whichmay be an embodiment of the transmitter portion of a transmitter system,e.g. such as transmitter system 110 in FIG. 1. In one embodiment, aseparate data rate and coding and modulation scheme may be used for eachof the N_(T) data streams to be transmitted on the N_(T) transmitantennas (i.e., separate coding and modulation on a per-antenna basis).The specific data rate and coding and modulation schemes to be used foreach transmit antenna may be determined based on controls provided byprocessor 130, and the data rates may be determined as described above.

Transmitter unit 200 includes, in one embodiment, a transmit dataprocessor 202 that receives, codes, and modulates each data stream inaccordance with a separate coding and modulation scheme to providemodulation symbols and transmit MIMO Transmit data processor 202 andtransmit data processor 204 are one embodiment of transmit dataprocessor 114 and transmit MIMO processor 120, respectively, of FIG. 1.

In one embodiment, as shown in FIG. 2, transmit data processor 202includes demultiplexer 210, N_(T) encoders 212 a through 212 t, andN_(T) channel interleavers 214 a through 214 t (i.e., one set ofdemultiplexers, encoders, and channel interleavers for each transmitantenna). Demultiplexer 210 demultiplexes data (i.e., the informationbits) into N_(T) data streams for the N_(T) transmit antennas to be usedfor data transmission. The N_(T) data streams may be associated withdifferent data rates, as determined by rate control functionality, whichin one embodiment may be provided by processor 130 or 170 (FIG. 1). Eachdata stream is provided to a respective encoder 212 a through 212 t.

Each encoder 212 a through 212 t receives and codes a respective datastream based on the specific coding scheme selected for that data streamto provide coded bits. In one embodiment, the coding may be used toincrease the reliability of data transmission. The coding scheme mayinclude in one embodiment any combination of cyclic redundancy check(CRC) coding, convolutional coding, Turbo coding, block coding, or thelike. The coded bits from each encoder 212 a through 212 t are thenprovided to a respective channel interleaver 214 a through 214 t, whichinterleaves the coded bits based on a particular interleaving scheme.The interleaving provides time diversity for the coded bits, permits thedata to be transmitted based on an average SNR for the transmissionchannels used for the data stream, combats fading, and further removescorrelation between coded bits used to form each modulation symbol.

The coded and interleaved bits from each channel interleaver 214 athrough 214 t are provided to a respective symbol mapping block 222 athrough 222 t, of transmit MIMO processor 204, which maps these bits toform modulation symbols.

The particular modulation scheme to be implemented by each symbolmapping block 222 a through 222 t is determined by the modulationcontrol provided by processor 130. Each symbol mapping block 222 athrough 222 t groups sets of q_(j) coded and interleaved bits to formnon-binary symbols, and further maps each non-binary symbol to aspecific point in a signal constellation corresponding to the selectedmodulation scheme (e.g., QPSK, M-PSK, M-QAM, or some other modulationscheme). Each mapped signal point corresponds to an M_(j)-ary modulationsymbol, where M_(j) corresponds to the specific modulation schemeselected for the j-th transmit antenna and M_(j)=2^(q) ^(j) . Symbolmapping blocks 222 a through 222 t then provide N_(T) streams ofmodulation symbols.

In the specific embodiment illustrated in FIG. 2, transmit MIMOprocessor 204 also includes a modulator 224 and inverse Fast Fouriertransform (IFFT) block 226 a through 226 t, along with symbol mappingblocks 222 a through 222 t. Modulator 224 modulates the samples to formthe modulation symbols for the N_(T) streams on the proper subbands andtransmit antennas. In addition modulator 224 provides each of the N_(T)symbol streams at a proscribed power level. In one embodiment, modulator224 may modulate symbols according to a hopping sequence controlled by aprocessor, e.g. processor 130 or 170. In such an embodiment, thefrequencies with which the N_(T) symbol streams are modulated may varyfor each group or block of symbols, frame, or portion of a frame of atransmission cycle.

Each IFFT block 226 a through 226 t receives a respective modulationsymbol stream from modulator 224. Each IFFT block 226 a through 226 tgroups sets of N_(F) modulation symbols to form corresponding modulationsymbol vectors, and converts each modulation symbol vector into itstime-domain representation (which is referred to as an OFDM symbol)using the inverse fast Fourier transform. IFFT blocks 226 a through 226t may be designed to perform the inverse transform on any number offrequency subchannels (e.g., 8, 16, 32, . . . , N_(F),).

Each time-domain representation of the modulation symbol vectorgenerated by IFFT blocks 226 a through 226 t is provided to anassociated cyclic prefix generator 228 a through 228 t. The cyclicprefix generators 228 a through 228 t pre-pending a prefix of a fixednumber of samples, which are generally a number of samples from the endof the OFDM symbol, to the N_(S) samples that constitute an OFDM symbolto form a corresponding transmission symbol. The prefix is designed toimprove performance against deleterious path effects such as channeldispersion caused by frequency selective fading. Cyclic prefixgenerators 228 a through 228 t then provide a stream of transmissionsymbols to an associated delay element 230 a through 230 t-1.

Each delay element 230 a through 230 t-1 provides a delay to each symbolthat is output from cyclic prefix generators 228 a through 228 t. In oneembodiment, the delay provided by each delay element 230 a through 230t-1 varies in time. In one embodiment, this delay is such that the delayvaries between consecutive symbols output by the cyclic prefix generatoror consecutive symbols that are to be consecutively transmitted from thetransmitter unit 200. In other embodiments, the delay may vary betweengroups of two, three, four, or more symbols with each symbol within thegroup having a same delay. In additional embodiments, all of the symbolsin a frame or burst period would have a same delay with each frame orburst period having a different delay for each symbol than a precedingor following frame or burst period.

Also, in the embodiment depicted in FIG. 2, the delay provided by eachdelay element 230 a through 230 t-1 is different than the delay providedby each other delay element. Further, while FIG. 2 depicts that cyclicprefix generator 228 a is not coupled to a delay element, otherembodiments may provide a delay element to the output of each of thecyclic prefix generators 228 a through 228 t.

The symbols output by delay elements 230 a through 230 t-1 are providedto an associated transmitter 232 a through 232 t which causes thesymbols to be transmitted by antennas 232 a through 232 t according tothe delay provided by delay elements 230 a through 230 t-1.

As stated above, in one embodiment, the time varying delay Δ_(m)provided by each delay element 230 a through 230 t- varies with time. Inone embodiment, the i-th OFDM symbol is transmitted as a transmittedsymbol from antenna m according a delay of Eq. 2: $\begin{matrix}{{s\left( {\left( {k - \Delta_{m}} \right)\quad{mod}\quad N} \right)} = {\frac{1}{\sqrt{N}}{\sum\limits_{n = 0}^{N - 1}{X_{n}{{\mathbb{e}}^{{- {j{({2{\pi/N}})}}}n\quad{\Delta_{m}{(i)}}} \cdot {\mathbb{e}}^{{- {j{({2{\pi/N}})}}}{nk}}}}}}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$The resulting overall channel in this case may be described as$\begin{matrix}{{H\left( {i,n} \right)} = {\sum\limits_{m = 1}^{M}{{H_{m}\left( {i,n} \right)} \cdot {\mathbb{e}}^{{- {j{({2{\pi/N}})}}}n\quad{\Delta_{m}{(i)}}}}}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$where H_(m)(i,n) is the channel n-th discrete Fourier Transform (DFT)coefficient for the channel impulse response from the m-th transmittingantenna to the receiving antenna.

The use of this time varying delay can introduce both frequencyselectivity and time selectivity into the channel that may be utilizedto improve performance. For example, by using time varying delays fortransmission symbols across different sub-carriers and different OFDMsymbols, both the time selectivity and frequency selectivity cansimultaneously be provided. Moreover, in the case of transmission tomultiple users, the time variation of the channel that is provided byvarying the delay for symbols can be exploited to provide diversitygains to each of the multiple users, since each user's receiver willhave different channel conditions than each other user's receiver.

In one embodiment, the delay Δ_(m)(i) may be changed in a linear fashionwith time with each consecutive, or group of consecutive symbols, beingdelayed by n*β samples, where β is a constant and n varying from 0, 1, .. . , N−1, where N is the number of symbols in a frame, burst period, orsymbol stream. In another embodiment, the delay Δ_(m)(i) may be a randomdelay, based upon a pseudo random sequence, with respect to an adjacentchannel, i.e. antenna, of the N_(T) antennas, a preceding and/orfollowing symbol. In an additional embodiment, the delay may be variedby f(x) where f is a function such as a sine, cosine, or other timevarying function, and x varies from 0, 1, . . . , N−1 or some multiplesthereof, where N is the number of symbols in a frame, burst period, orsymbol stream. In each of the previous embodiments, the delay may alsobe changed based on feedback information, in which case the receiversends back a channel quality indicator that describes the overallchannel conditions and Δ_(m)(i) is changed to improve the overallquality.

Referring to FIG. 3, an embodiment of a time varying delay applied tosymbols transmitted from a same antenna is illustrated. Symbols S₁, S₂,S₃, and S₄ are generated to be transmitted during consecutive time slotsT₁, T₂, T₃, and T₄ respectively. Each symbol S₁, S₂, S₃, and S₄comprises nine samples N_(S1), N_(S2), N_(S3), N_(S4), N_(S5), N_(S6),N_(S7), N_(S8), N_(S9), and a two-sample cyclic prefix N_(C1) andN_(C2), which are samples N_(S8), and N_(S9) respectively. It should benoted that the content of each sample may be different for each of thesymbols. It should be noted that the samples N_(S1), N_(S2), N_(S3),N_(S4), N_(S5), N_(S6), N_(S7), N_(S8), N_(S9) are to be combined toform symbol S₁ in the order of N_(S1), N_(S2), N_(S3), N_(S4), N_(S5),N_(S6), N_(S7), N_(S8), N_(S9).

The delay element, e.g. delay element 230 a, then provides a delay tothe symbol S₁, S₂, S₃, and S₄ that are transmitted from the sameantenna. In the embodiment depicted in FIG. 3, the delay for symbol S₁is one sample period t₁. The next symbol S₂, which is to be transmittedon the same antenna immediately after symbol S₁, is delayed by twosample periods t₁ and t₂. The next symbol S₃, which is to be transmittedon the same antenna immediately after symbol S₂, is delayed by threesample periods t₁, t₂, and t₃. The next symbol S₄, which is to betransmitted on the same antenna immediately after symbol S₃, is delayedby four sample periods t₁, t₂, t₃, and t₄. If additional symbols are tobe transmitted on the same antenna, the next consecutive symbol would betransmitted with a delay of five sample periods t₁, t₂, t₃, t₄, and t₅.In this way, a linear time varying delay may be applied to transmissionfrom an antenna, which may or may not be part of a MIMO system.

It should be noted the linear variation of the delay period need not besequential by one sample period, but may be sequential by 2 or moresample periods as well, e.g. the first symbol S₁ may be delayed by threesample periods, the second symbol S₂ is delayed by six sample periods,the third symbol S₃ is delayed by nine sample periods, and the fourthsymbol S₄ is delayed by twelve sample periods. Also, the linearvariation need not vary between each consecutive symbol but for groupsof symbols, e.g. symbols S₁ and S₂ are each delayed by one sample periodand symbols S₃, and S₄ are each delayed by two or more sample periods.

Referring to FIG. 4, an embodiment of a time varying delay applied to asymbol transmitted on multiple antennas is illustrated. A same symbol S₁is to be transmitted from antennas A₁, A₂, A₃, and A₄. Symbol S₁comprises nine samples N_(S1), N_(S2), N_(S3), N_(S4), N_(S5), N_(S6),N_(S7), N_(S8), N_(S9), and a two-sample cyclic prefix N_(C1) andN_(C2), which are samples N_(S), and N_(S9) respectively. From firstantenna A₁ symbol S₁ is not delayed by any sample periods. From secondantenna A₂, symbol S₁ is delayed by one sample period t₁. From thirdantenna A₃, symbol S₁ is delayed by two sample periods t₁ and t₂. Fromfourth antenna A₄, symbol S₁ is delayed by three sample periods t₁, t₂,and t₃. As such, time and frequency diversity may be provided in a MIMOsystem in addition to the spatial diversity provided by antennas A₁, A₂,A₃, and A₄.

The time diversity provided for the scheme depicted in FIG. 4, andvariations thereof, provides a reduction in the likelihood of collisionsby the same samples of a same symbol thereby minimizing the possibilityof destructive or constructive addition of the channels.

It should be noted that the delay variation between a same symboltransmitted on a same antenna need not be linear or even related to thedelay on the other antennas, so long as if the symbol is to betransmitted substantially simultaneously, it should be delayed by adifferent amount on each antenna.

It should be noted that, the order utilized need not correspond to thenumber of antennas and may vary for smaller groups or in a larger numberthan the number of antennas.

In addition, as discussed with respect to FIG. 2, the delay may berandom and may be based on a function such as a sine, cosine, or otherfunction. In some embodiments, the delay period is limited to a numberof samples in a symbol, where the delay period may be repeated after afixed or random number of symbols. Also, it should be noted that thedelay between symbols can be fractions of sample periods and is notlimited to being multiples of entire sample periods. The fractionaldelay may be implemented, in one embodiment, by using fractions of clockperiods of the one or more clocks of transmitter unit 200.

Referring to FIG. 5, a block diagram of another embodiment of atransmitter unit that provides time-varying delay diversity isillustrated. Transmitter unit 500 is substantially identical totransmitter unit 200. In addition, scaling circuits 554 a through 554t-1 are each coupled to an output of one of the delay elements 530 athrough 530 t-1. Scaling circuits 534 a through 534 t-1 provide a fixedscalar shift to the delay provided by each of the delay elements 530 athrough 530 t-1. For example, a fixed shift is applied to each delay sothat, for example if a constant shift of 0.5 is applied then a onesample period delay would be 0.5 samples periods, a two-sample perioddelay would be one sample period, and a five sample period delay wouldbe two and a half sample periods. In one embodiment, each of scalingcircuits 554 a through 554 t-1 provides a shift that is different thaneach other scaling circuit. In one embodiment, a linear progression isprovided across scaling circuits 554 a through 534 t-1 that is scalingcircuit 554 a provides a shift less than, 554 b, which is less than 554c, etc.

It should be noted that while FIG. 5, depicts that cyclic prefixgenerator 228 a is not coupled to a delay element, other embodiments mayprovide a delay element to the output of each of the cyclic prefixgenerators 228 a through 228 t. Also, while FIG. 5, depicts that cyclicprefix generator 228 a is not coupled to a scaling circuit, otherembodiments may provide a scaling circuit to the output of each of thecyclic prefix generators 228 a through 228 t, regardless of whether adelay circuit is coupled to the cyclic prefix generator.

Referring to FIG. 6, a block diagram of an embodiment of a receiver unitcapable of utilizing time-varying delay diversity is illustrated. Thetransmitted signals are received by antennas 602 a through 602 r andprocessed by receivers 604 a through 604 r, respectively, to provideN_(R) sample streams, which are then provided to an RX processor 606.

Within demodulator 608, cyclic prefix removal element 612 a through 612r and FFT blocks 614 a through 614 r provide N_(R) symbol streams.Cyclic prefix removal elements 612 a through 612 r remove the cyclicprefix included in each transmission symbol to provide a correspondingrecovered OFDM symbol.

FFT blocks 614 a through 614 r then transform each recovered symbol ofthe symbol stream using the fast Fourier transform to provide a vectorof N_(F) recovered modulation symbols for the N_(F) frequencysubchannels for each transmission symbol period. FFT blocks 614 athrough 614 r provide N_(R) received symbol streams to a spatialprocessor 620.

Spatial processor 620 performs spatial or space-time processing on theN_(R) received symbol streams to provide N_(T) detected symbol streams,which are estimates of the N_(T) transmitted symbol streams. Spatialprocessor 620 may implement a linear ZF equalizer, a channel correlationmatrix inversion (CCMI) equalizer, a minimum mean square error (MMSE)equalizer, an MMSE linear equalizer (MMSE-LE), a decision feedbackequalizer (DFE), or some other equalizer, which are described anddepicted in U.S. patent application Ser. Nos. 09/993,087, 09/854,235,09/826,481, and 09/956,444 each of which is incorporated herein byreference in their entireties.

Spatial processor 620 may be capable of compensating for the timevarying delay provided by the delay elements and/or scaling circuit ofthe transmitters, which are discussed with respect to FIGS. 2 and 5.This compensation may be provided, in one embodiment, by having thedelay scheme, e.g. linear, random according to a pseudo random sequence,or function, known a priori by the receiver unit 600. This knowledge maybe provide, for example, by having a same scheme utilized by alltransmitters or providing information as to the scheme utilized as partof the initialization of communication between the transmitter andreceiver unit 600.

A multiplexer/demultiplexer 622 then multiplexes/demultiplexes thedetected symbols, and provides N_(D) aggregated detected symbol streamsfor the N_(D) data streams to N_(D) symbol demapping elements 624 athrough 624 r. Each symbol demapping element 624 a through 624 r thendemodulates the detected symbols in accordance with a demodulationscheme that is complementary to the modulation scheme used for the datastream. The N_(D) demodulated data streams from N_(D) symbol demappingelements 624 a through 624 r are then provided to a RX data processor610.

Within RX data processor 610, each demodulated data stream isde-interleaved by a channel de-interleaver 632 a through 632 r in amanner complementary to that performed at the transmitter system for thedata stream, and the de-interleaved data is further decoded by a decoder634 a through 634 r in a manner complementary to that performed at thetransmitter system. For example, a Turbo decoder or a Viterbi decodermay be used for decoder 634 a through 634 r if Turbo or convolutionalcoding, respectively, is performed at the transmitter unit. The decodeddata stream from each decoder 634 a through 634 r represents an estimateof the transmitted data stream. Decoders 634 a through 634 r may alsoprovide the status of each received packet (e.g., indicating whether itwas received correctly or in error). Decoder 634 a through 634 r mayfurther store demodulated data for packets not decoded correctly, sothat this data may be combined with data from a subsequent incrementaltransmission and decoded.

In the embodiment shown in FIG. 6, a channel estimator 640 estimates thechannel response and the noise variance and provides these estimates toprocessor 650. The channel response and noise variance may be estimatedbased on the detected symbols for the pilot.

Processor 650 may be designed to perform various functions related torate selection. For example, processor 650 may determine the maximumdata rate that may be used for each data stream based on the channelestimates and other parameters such as the modulation scheme.

Referring to FIG. 7, a block diagram of an embodiment of a delay elementis illustrated. A processor 700 is coupled via a bus 702 with a memory704. Memory 704 is utilized to store samples of time-domainrepresentations of the modulation symbols that are provided fortransmission. The samples of each symbol are stored in memory locationsthat are known to processor 700. Processor 700 can then instruct memory704 to output the samples of each symbol utilizing any desired timevarying delay for consecutive, groups, or frames or burst periods ofsymbols.

As described with respect to FIGS. 3 and 4, the delay for each symbolmay vary between each symbol, for groups of symbols that are to betransmitted contiguously, and between symbols in different frames orbursts periods. The use of a memory allows for any predetermined oradaptive scheme to be utilized for delaying provision of the symbols andtherefore providing time diversity that may be varied based upon channelconditions as well as pre-determined schemes, e.g. a linear variation.

Referring to FIG. 8, a flow chart of an embodiment of a method forproviding time-varying delay diversity is illustrated. Samples of thatrepresent one or more modulated symbols after being subject to aninverse Fast Fourier transform are provided, block 800. A cyclic prefixis then pre-pended to each of the modulated symbols, block 802. The sizeof the prefix may vary as desired, and in one embodiment may bethirty-two or more samples.

The samples including the cyclic prefix are then stored in a memory,block 804, which in one embodiment may be a buffer. In one embodiment,the samples for each modulated symbol are stored in memory according tothe order in which they are provided after pre-pending of the cyclicprefix. In other embodiments, the samples of each modulated symbol maybe stored in any order that is desired. A first symbol to be transmittedfor a symbol is removed according to a first delay N, block 806. Thenext symbol to be transmitted is then removed according a second delay,which is different than the first delay, block 808. The second delay,and additional delays for later symbols, may be delays of N+β, where βmay be a linear increase or decrease from N, a random change from N, orbe the result of some function.

A determination is then made as to whether of the symbols that need tobe delayed have been transmitted or otherwise utilized, block 810. Ifnot, additional delays are provided to the output of further symbolsfrom the memory or buffer according to the same time variance, block808. If yes, then the process ceases, and additional symbols areprovided as discussed with respect blocks 800-804, block 812.

Referring to FIG. 9, a block diagram of a further embodiment of atransmitter unit that provides time-varying delay diversity isillustrated. Transmitter unit 900 is substantially identical totransmitter unit 200. However, instead of utilizing delay element 230 athrough 230 t-1 coupled to an output of IFFT blocks 226 a through 226 t,phase shift circuits 930 a through 930 t-1 are coupled before an inputof IFFT blocks 926 a through 926 t, in they receive the output ofmodulator 924. Phase shift circuits 930 a through 930 t-provide atime-varying phase shift to the samples of each symbol. For example,phase shift circuit 930 a may provide a phase shift φ₁ to the samples ofa first symbol and a phase shift φ₁ to the samples of a next or latersymbol output by the modulator. The samples of a later symbol may have aphase shift of a different or same amount. This phase shift, may operateas a delay in the time domain, after application of the IFFT by IFFTblocks 926 a through 926 t.

Each phase shift circuit 930 a through 930 t may provide a differentphase shift than each other phase shift circuit 930 a through 930 t, sothat a delay of a same symbol transmitted from multiple antennas isdifferent from each antenna. The variance may or may not be a functionof the phase shift applied with respect to any other antenna.

In one embodiment, the phase shift provided by each phase shift circuit930 a through 930 t is such that the phase shift varies betweenconsecutive symbols output by the modulator. In other embodiments, thephase shift may vary between groups of two, three, four, or more symbolswith each symbol within the group having a same phase shift. Inadditional embodiments, all of the symbols in a frame or burst periodwould have a same phase shift with each frame or burst period having adifferent phase shift for each symbol than a preceding or followingframe or burst period.

It should be noted that while FIG. 9, depicts that a phase shift circuitis not coupled to cyclic prefix generator 928 a, other embodiments mayprovide a phase shift circuit to the output of each of the cyclic prefixgenerators 928 a through 928 t.

In some embodiment, the modulator and the phase shift circuit maycomprise a processor.

Referring to FIG. 10, a flow chart of an embodiment of a method forproviding time-varying delay diversity is illustrated. Samples of afirst symbol output by a modulator is subject to a first phase shift φ₁,block 1000. The samples of a second symbol are then subject to a phaseshift φ₂, which is different than φ₁, block 1002. A determination isthen made whether additional symbols have yet to be phase shifted, block1004. If not, then a phase shift which may be the same or different asφ₁ or φ₂ is then applied to the samples of a next symbol output by themodulator, block 106. This process is then repeated, until there are noadditional symbols to phase shift.

If there are no additional symbols to phase shift, the symbols aresubject to IFFT, block 1008, the pre-pending of a cyclic prefix, block1010, and stores the symbol in memory, block 1012, which in oneembodiment may be a buffer. In one embodiment, the samples for eachmodulated symbol are stored in memory according to the order in whichthey are provided after pre-pending of the cyclic prefix. In otherembodiments, the samples of each modulated symbol may be stored in anyorder that is desired.

In some embodiments, the phase shift may be different betweenconsecutive symbols, groups of symbols, or frames by a phase that isequal to a constant angle multiplied by a varying number correspondingto the location of the symbol in a symbol stream or other ordinalnumber. The constant angle may be fixed or may vary according to othertime constants. Also, the constant angle applied to symbols intended fordifferent antennas may be different.

In other embodiments, the phase shift may vary according to a randomphase with respect to any other symbols. This may be provided byutilizing a pseudo random code to generate the phase shift.

It should be noted that while FIG. 10 illustrates waiting to performIFFT and cyclic prefix pre-pending until a phase shift is applied to allof the symbols of a frame or burst period, each symbol may be subject toIFFT and cyclic prefix pre-pending either individually or in groupsbefore completion of phase shifts to each of the symbols of a frame orburst period.

It should be noted that transmitter 200 and 500 may receive and processa respective modulation symbol stream (for MIMO without OFDM) ortransmission symbol stream (for MIMO with OFDM) to generate a modulatedsignal, which is then transmitted from the associated antenna. Otherdesigns for the transmitter unit may also be implemented and are withinthe scope of the invention.

The coding and modulation for MIMO systems with and without OFDM aredescribed in further detail in the following U.S. patent applications:

-   -   U.S. patent application Ser. No. 09/993,087, entitled        “Multiple-Access Multiple-Input Multiple-Output (MIMO)        Communication System,” filed Nov. 6, 2001;    -   U.S. patent application Ser. No. 09/854,235, entitled “Method        and Apparatus for Processing Data in a Multiple-Input        Multiple-Output (MIMO) Communication System Utilizing Channel        State Information,” filed May 11, 2001;    -   U.S. patent application Ser. Nos. 09/826,481 and 09/956,449,        both entitled “Method and Apparatus for Utilizing Channel State        Information in a Wireless Communication System,” respectively        filed Mar. 23, 2001 and Sep. 18, 2001;    -   U.S. patent application Ser. No. 09/776,075, entitled “Coding        Scheme for a Wireless Communication System,” filed Feb. 1, 2001;        and    -   U.S. patent application Ser. No. 09/532,492, entitled “High        Efficiency, High Performance Communications System Employing        Multi-Carrier Modulation,” filed Mar. 30, 2000.        These applications are all assigned to the assignee of the        present application and incorporated herein by reference.        Application Ser. No. 09/776,075 describes a coding scheme        whereby different rates may be achieved by coding the data with        the same base code (e.g., a convolutional or Turbo code) and        adjusting the puncturing to achieve the desired rate. Other        coding and modulation schemes may also be used, and this is        within the scope of the invention.

Those skilled in the art will appreciate that the various illustrativelogical blocks, modules, circuits, and algorithms described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, andalgorithms have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, processors, modules, andcircuits described in connection with the embodiments disclosed hereinmay be implemented or performed with a general purpose processor, adigital signal processor (DSP), circuits, an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general purpose processor maybe a microprocessor, but in the alternative, the processor may be anyconventional processor, processor, microprocessor, or state machine. Aprocessor may also be implemented as a combination of devices, e.g., acombination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, multiple logic elements, multiple circuits, or any other suchconfiguration.

The methods or algorithms described in connection with the embodimentsdisclosed herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for providing transmission diversity in a multi-channelcommunication system, comprising: providing, to a first antenna, a firstsymbol comprising a first plurality of samples; providing, to a secondantenna, the first symbol after a first delay period; providing, to thefirst antenna, a second symbol comprising a second plurality of samples;providing, to the second antenna, the second symbol after a second delayperiod that is different than the first delay period; providing, to thefirst antenna, a third symbol comprising a third plurality of samples;providing, to the second antenna, the third symbol after a third delayperiod that is different than the first delay period and the seconddelay period.
 2. The method of claim 1, wherein the first symbol, secondsymbol, and third symbol are consecutive symbols of a same symbolstream.
 3. The method of claim 1, wherein the third symbol istransmitted immediately after the second symbol that is transmittedimmediately after the first symbol.
 4. The method of claim 1, whereinthe first symbol, second symbol, and third symbol are non-consecutivesymbols of a same symbol stream.
 5. The method of claim 4, wherein thefirst symbol and second symbol are separated by a number of symbols andthe second symbol and the third symbol are separated by the number ofsymbols.
 6. The method of claim 1, wherein the first symbol, secondsymbol, and third symbol are each transmitted in different frames. 7.The method of claim 1, wherein providing the first symbol after thefirst delay period comprises applying a first phase shift to each of thefirst plurality of samples; providing after the second symbol after thesecond delay period that is different than the first delay periodcomprises applying a second phase shift to the second plurality ofsamples, that is different than the first phase shift; and providing thethird symbol after a third delay period that is different than the firstdelay period and the second delay period comprises applying a thirdphase shift to the third plurality of samples that is different than thefirst phase shift and the second phase shift.
 8. The method of claim 1,wherein the first delay period comprises a delay of X sample periods,the second delay period comprises a delay of X+β sample periods, and thethird delay comprises a delay of X+2β sample periods.
 9. The method ofclaim 1, wherein the first delay period comprises a delay of X sampleperiods, the second delay period comprises a delay of Y sample periodswhich is any number of sample periods other than X, and the third delaycomprises a delay of Z sample periods which is any number of sampleperiods other than X or Y.
 10. The method of claim 1, wherein the firstdelay period comprises a delay of f(x) sample periods, the second delayperiod comprises a delay of f(x+1) sample periods, and the third delaycomprises a delay of f(x+1) sample periods, where f is a function. 11.The method of claim 1, further comprising modulating the first symbol,second symbol, and third symbol according to different carrierfrequencies.
 12. The method of claim 1, further comprising providing ascalar shift to the first delay period, second delay period, and thirdperiod.
 13. The method of claim 1, wherein the first delay period, thesecond delay period, and the third delay period are determined basedupon feedback information.
 14. The method of claim 1, further comprisingappending, prior to providing, a cyclic prefix to the first symbol;appending, prior to providing, a cyclic prefix to the second symbol; andappending, prior to providing, a cyclic prefix to the third symbol. 15.A transmitter comprising: at least two antennas; a modulator coupledwith the at least two antennas; and a delay circuit coupled between themodulator and one of the at least two antennas, the delay circuitdelaying symbols output from the modulator to the antenna by a delayperiod that varies over time.
 16. The transmitter of claim 15, whereinthe delay circuit provides the delay period so that it is differentbetween consecutive symbols output by the modulator to the antenna. 17.The transmitter of claim 15, wherein the delay circuit provides thedelay period so that it is different between consecutive symbols by anumber of sample periods that is equal to a constant multiplied by avarying number corresponding to the location of the symbol.
 18. Thetransmitter of claim 15, wherein the delay circuit provides the delayperiod so that it is different between consecutive symbols by a randomnumber of sample periods with respect to any other consecutive symbols.19. The transmitter of claim 15, wherein the delay circuit provides thedelay period so that it is different between consecutive symbols by f(x)sample periods, where f is a function and x is a number that variesaccording to the location of the symbol.
 20. The transmitter of claim15, further comprising a scaling circuit coupled between the delaycircuit and the antenna.
 21. The transmitter of claim 15, wherein themodulator utilizes different carrier frequencies of a frequency band tomodulate symbols.
 22. The transmitter of claim 15, wherein the modulatorand the delay circuit comprise a processor.
 23. The transmitter of claim15, wherein the at least two antennas comprise at least three antennas,the transmitter further comprising another delay circuit coupled betweenthe modulator and another of the at least three antennas, the anotherdelay circuit providing another delay period that varies over time,wherein the another delay period and the delay period are different foreach symbol.
 24. The transmitter of claim 23, wherein the delay periodis different between consecutive symbols output by the modulator to theantenna and the another delay period is different between consecutivesymbols output by the modulator to the another antenna, and wherein thedelay period between consecutive symbols varies by n*β samples and theanother delay period between consecutive symbols varies by n*(β+k)samples.
 25. The transmitter of claim 25, further comprising a cyclicprefix generator circuit coupled between the modulator and the delaycircuit.
 26. A wireless transmitter, comprising: at least two antennas;and a memory coupled with the antenna, the memory storing a plurality ofsymbols each comprising a plurality of samples, wherein the memoryoutputs the plurality of samples of a first symbol of the plurality ofsymbols after a first delay to one antenna of the at least two antennasand a second symbol of the plurality of symbols after a second delay tothe one antenna, wherein the first delay and the second delay aredifferent.
 27. The wireless transmitter of claim 26, wherein the seconddelay is n*β samples greater than the first delay, where n is an ordinallocation of the second symbol in a symbol stream and β is a constant.28. The wireless transmitter of claim 26, wherein the first delay isf(x) samples and the second delay is f(x+1) samples, where f is afunction.
 29. The wireless transmitter of claim 26, wherein the memorycomprises a buffer.
 30. The wireless transmitter of claim 26, furthercomprising a cyclic prefix generator coupled with the memory that adds acyclic prefix to each of the plurality of symbols prior to storing theplurality of symbols in memory.
 31. The wireless transmitter of claim26, wherein the at least two antennas comprise at least three antennas,wherein the memory provides the first symbol to another of the at leastthree antennas after a third delay and provides the second symbol aftera fourth delay to the another antenna, wherein the first delay, seconddelay, third delay, and fourth delay are all different from each other.32. The wireless transmitter of claim 26, wherein the second delayvaries by n*β samples from the first delay and the fourth delay variesby n*(β+k) samples from the third delay, where n is a location of asymbol in a symbol stream and β and k are constants.
 33. A transmitterfor transmitting a plurality of symbols, comprising: a plurality ofantennas; a modulator coupled to the plurality of antennas thatmodulates a plurality of symbols for transmission over the plurality ofantennas; and means for providing a delay of each symbol output by themodulator and for varying the delay between consecutive symbols outputby the modulator.
 34. The transmitter of claim 33, wherein the means forproviding comprises means for providing a delay between consecutivesymbols that varies by n*β samples, where n is an ordinal location ofthe second symbol in a symbol stream and β is a constant.
 35. Thetransmitter of claim 33, wherein the means for providing comprises meansfor providing a delay between consecutive symbols that varies by arandom number of samples.
 36. The transmitter of claim 33, wherein themeans for providing comprises means for providing a delay betweenconsecutive symbols that varies by f(x) samples, where f is a function.37. The transmitter of claim 33, further comprising a modulator thatmodulates the plurality of symbols utilizing a plurality of carrierfrequencies.
 38. The transmitter of claim 33, further comprising meansfor providing a scalar shift to the delay provided to each of theplurality of symbols.
 39. The transmitter of claim 33, furthercomprising means for appending a cyclic prefix of N samples to each ofthe plurality of symbols.
 40. A transmitter comprising: at least threeantennas; a modulator coupled to the at least three antennas; a firstdelay circuit coupled between the modulator and one of the at least twoantennas, the first delay circuit delaying symbols output from themodulator to the antenna by a delay period that varies over time; asecond delay circuit coupled between the modulator and another of the atleast two antennas, the first delay circuit delaying symbols output fromthe modulator to the another antenna by a another delay period thatvaries over time, wherein the another delay period and the delay periodare different.
 41. The transmitter of claim 40, wherein the delaycircuit provides the delay period so that it is different betweenconsecutive symbols output by the modulator to the antenna and theanother delay circuit provides the another delay period so that it isdifferent between consecutive symbols output by the modulator to theanother antenna.
 42. The transmitter of claim 40, wherein the delaycircuit provides the delay period so that it is different betweenconsecutive symbols by a number of sample periods that is equal to aconstant multiplied by a varying number corresponding to the location ofthe symbol and the another delay circuit provides the another delayperiod so that it is different between consecutive symbols by a numberof sample periods that is equal to another constant multiplied by avarying number corresponding to the location of the symbol.
 43. Thetransmitter of claim 40, wherein the another delay circuit provides thedelay period so that it is different between consecutive symbols by arandom number of sample periods with respect to any other consecutivesymbols.
 44. The transmitter of claim 40, wherein the delay circuitprovides the delay period so that it is different between consecutivesymbols by f(x) sample periods, where f is a function and x is a numberthat varies according to the location of the symbol and the anotherdelay circuit provides the another delay period so that it is differentbetween consecutive symbols by g(x) sample periods, where g is afunction different than f and x is a number that varies according to thelocation of the symbol.
 45. The transmitter of claim 40, furthercomprising a scaling circuit coupled between the delay circuit and theantenna and another scaling circuit coupled between the another delaycircuit and the another antenna.
 46. The transmitter of claim 40,wherein the modulator utilizes different carrier frequencies of afrequency band to modulate symbols.
 47. The transmitter of claim 40,further comprising a cyclic prefix generator circuit coupled betweenmodulator and the delay circuit and another cyclic prefix generatorcoupled between the modulator and the another delay circuit.
 48. Amethod for providing transmission diversity in a multi-channelcommunication system, comprising: applying a first phase shift to afirst symbol to be transmitted on a first antenna, the first symbolcomprising a first plurality of samples; and applying a second phaseshift, different than the first phase shift, to the first symbol to betransmitted on a second antenna, the second symbol comprising a secondplurality of samples.
 49. The method of claim 48, further comprisingapplying a third phase shift to a second symbol to be transmitted on thefirst antenna and applying a fourth phase shift to the second symbol tobe transmitted on the second antenna, wherein the first, second, third,and fourth phase shifts are all different from each other.
 50. Themethod of claim 49, wherein the second symbol is transmitted immediatelyafter the first symbol.
 51. The method of claim 49, wherein the firstsymbol and second symbol are non-consecutive symbols of a same symbolstream.
 52. The method of claim 49, wherein the first symbol and secondsymbol are each transmitted in different frames.
 53. The method of claim49, further comprising modulating the first symbol and second symbolaccording to different carrier frequencies.
 54. The method of claim 48,wherein the first phase shift and the second phase shift are determinedbased upon feedback information.
 55. The method of claim 48, furthercomprising applying an IFFT operation to the first symbol prior totransmission.
 56. A transmitter comprising: at least two antennas; amodulator coupled with the at least two antennas; and a phase shiftcircuit coupled between the modulator and one of the at least twoantennas, the phase shift circuit applying a phase shift to symbolsoutput by the modulator to the antenna by a phase shift that varies overtime.
 57. The transmitter of claim 56, wherein the phase shift circuitprovides the phase shift so that it is different between consecutivesymbols output by the modulator to the antenna.
 58. The transmitter ofclaim 56, wherein the phase shift circuit provides the phase shift sothat it is different between consecutive symbols by a phase that isequal to a constant angle multiplied by a varying number correspondingto the location of the symbol.
 59. The transmitter of claim 56, whereinthe phase shift circuit provides the phase shift so that it is differentbetween symbols by a random phase with respect to any other symbols. 60.The transmitter of claim 56, wherein the modulator utilizes differentcarrier frequencies of a frequency band to modulate symbols.
 61. Thetransmitter of claim 56, wherein the modulator and the phase shiftcircuit comprise a processor.
 62. The transmitter of claim 56, whereinthe at least two antennas comprise at least three antennas, thetransmitter further comprising another phase shift circuit coupledbetween the modulator and another of the at least three antennas, theanother phase shift circuit providing another phase shift that variesover time, wherein the another phase shift and the shift are differentfor each symbol.
 63. The transmitter of claim 56, further comprising anIFFT block coupled to an output of the phase shift circuit.